The present invention relates, in general to radio frequency modulation techniques, and more particularly, to synchronous demodulation of radio frequency signals.
The recent rise-in popularity and use of contactless credit cards or smartcards emphasizes the importance of having effective and reliable operation of such smartcard systems. A typical smartcard system has a smartcard terminal or reader and a portable smartcard that is powered by the reader through magnetic fields coupled through an antenna on the reader and an antenna on the smartcard. The reader typically transmits a radio frequency signal that is received by the smartcard and is used to power electronics within the smartcard. The smartcard recovers a clock from the radio frequency signal and uses the clock to form a subcarrier used to modulate data onto the radio frequency signal. This modulated radio frequency signal is received by the reader. The reader demodulates the modulated radio frequency signal to extract the encoded data. In the past, various techniques have been used to demodulate the encoded data transmitted by the smartcard to the reader.
Typically, the reader transmits the RF carrier through an output antenna. Additionally, the reader has a carrier rejection tuned circuit attached to the antenna of the reader in order to receive the data modulated RF carrier. This arrangement requires critical and precise tuning because rejection of the power in the carrier inherently requires a high xe2x80x9cQxe2x80x9d filter. This increases the cost of the reader system. Over time, the frequency adjustment drifts, thus, constant readjustment is required to ensure proper operation.
Additionally, the carrier rejection tuned circuit only passes one sideband of the double sidebands in the received RF signal. This results in a loss of at least 6 dB, thus, the amplitude of the RF signal recovered by the tuned circuit is very small, typically less than approximately 2.0 milli-volts. Consequently there is a small signal-to-noise ratio making it difficult to accurately recover the data from the modulated RF carrier.
Further, because the carrier rejection tuned circuit only recovers one sideband, it creates a phase modulation factor that makes it difficult to recover the phase of the received modulated RF signal, thus, making it difficult to recover data transmitted by the smartcard to the reader
Accordingly, it is desirable to have a demodulation method that does not require a critical adjustment to tune to the RF carrier frequency, that does not create phase modulation, and that recovers a large signal from the transmitted signal from the card.